1. Field of the Invention
The present invention relates to a method and an apparatus for optically measuring or determining an amount of an electric current or strength of a magnetic field, by utilizing the Faraday effect. More particularly, the invention is concerned with such method and apparatus for measuring an amount of electric current which flows through an electrically conductive material such as electric power transmission or distribution lines.
2. Discussion of the Prior Art
In the art of measuring the strength of a magnetic field, a technique utilizing the Faraday effect, i.e., a magnetooptical effect, is drawing an increasing attention of the industries concerned. This technique uses a light beam as a medium for the measurement, and consequently, assures a high degree of insulation from electromagnetic induction noises. Application of this feature has been proposed to measure an electric current which produces a magnetic field.
Such a magnetooptical technique which utilizes the Faraday effect is generally practiced by an apparatus which includes a magnetooptical sensing head having a Faraday-effect element or magnetooptical element, a polarizer and an analyzer, a light source device for generating a light beam to which the sensing head is exposed, and a light-sensitive means for converting an optical output of the sensing head into an electric signal. The sensing head modulates the incident light beam according to the strength of a magnetic field to which the sensing head is exposed. The thus modulated light beam is received by the light-sensitive means.
In the apparatus indicated above, the light beam produced by the light source device is linearly polarized by the polarizer of the sensing head, and the linearly polarized light beam is transmitted through the magnetooptical or Faraday element in the magnetic field. As a result, the plane of polarization of the linearly polarized light beam is rotated as a function of the magnetic field strength, due to the Faraday effect. The light beam whose polarization plane is rotated by the magnetooptical element is incident upon the analyzer which has a polarizing direction different from that of the polarizer, whereby the angle of rotation of the polarization plane is converted into the amount of light which has passed through the analyzer. In other words, a change in the optical output of the sensing head corresponds to a change in the strength of the magnetic field to which the sensing head is exposed. As is well known in the art, the optical output of the sensing head is expressed by a formula which includes the Verdet's constant. According to this formula, the strength of the magnetic field, and the amount of electric current which gives that magnetic field strength, may be determined.
However, it is recognized that the magnetooptical current/magnetic field measuring apparatus utilizing the Faraday effect suffers from a potential problem that the output signal level is influenced by a change in the environments of the apparatus. For instance, the above-indicated formula used to determine the magnetic field strength from the optical output includes the Verdet's constant of the material of the magnetooptical element (Faraday-effect element) which has a certain degree of temperature dependence. While various solutions to this problem have been considered and proposed, none of them have been found satisfactory or sufficiently effective to solve the problem, and some of the proposed solutions require a complicated arrangement or result in an increase in the size of the apparatus. Thus, the proposed solutions are not practically available.